New concerns on insect resistance

The fear is that growers not adhering to resistance management guidelines to delay use of pyrethroids until after bloom in Mississippi, Arkansas and Louisiana may be increasing the potential for resistance development in plant bug, cotton aphid and bollworm populations.

A high level of resistance in bollworm, should it occur, would force growers to shift to higher-cost compounds to control bollworm escapes in Bt cotton, which may give growers pause as to the cost-effectiveness of the worm-resistant technology.

The loss of pyrethroids for cotton would also mean the loss of a cost-effective product for worm control in other crops.

At one time, pyrethroids were the preferred product for tobacco budworm and bollworm control in cotton. Tobacco budworm first started showing resistance to pyrethroids in the early 1980s. By the mid-1980s, resistance reached levels which concerned entomologists.

“There was a real urgency to maintain pyrethroids when resistance started cropping up with tobacco budworm because there were no other tools for effective control.” said Scott Martin, with Syngenta, and chairman of the chemical industry’s Insecticide Resistance Action Committee (IRAC).

But after Bt cotton, which is highly effective against tobacco budworms, “there was less concern about resistance in other pests that the various resistance management plans addressed.”

The result of this has been more use of pyrethroids out of recommended spraying guidelines which has led to an increase in resistance levels in several pest species.

“We know we already have resistance in some tarnished plant bug populations from early-season use of pyrethroids,” Martin said. “From the information I have, we have only one effective pyrethroid spray on tarnished plant bug. After that, they’re no longer effective. If you use pyrethroid early on for something else and you expose the plant bug, then you lose an effective spray later on.”

Bollworm is another pest that IRAC is watching closely, according to Martin. “As a group, IRAC sponsored a monitoring program throughout the Mid-South, Southeast and Texas. It’s shown that there’s been a change in bollworm susceptibility to pyrethroids in certain areas. In South Carolina, we’ve heard reports of bollworm resistance developing.”

In Louisiana, “we have not been able to document any populations of bollworm that are resistant to pyrethroids at a level that would constitute failures to attain satisfactory control in cotton fields,” said entomologist Roger Leonard, of LSU AgCenter’s Macon Ridge Research Station.

But he’s quick to point out that it’s possible. And that’s a key point for growers.

“If you lose pyrethroids for bollworms, the next line of products that we have to use will cost 2- 3-fold more. At that point, you begin to question the value of Bollgard. If you have to spray Bollgard cotton with Tracer, Denim or Steward for bollworms, the value of Bollgard is compromised.

“From another standpoint, pyrethroids are used on all sorts of crops -- sweet corn, tomato, soybeans. If these products become ineffective on cotton, bollworm control on other hosts may become less than satisfactory. That’s the really important message. We have to have that class of chemistry for corn earworm, bollworm, sorghum head worm or soybean podworm.”

Given the level of pyrethroid resistance that tobacco budworms have, many growers still include pyrethroids as control options, Leonard said. “Because they’re so inexpensive, producers tank mix products with them and try to get by with it.

“Not every tobacco budworm out there is resistant,” he added. “You’re going to kill a few of them. And in situations where you have very low pressure, you may sneak by once or twice. But entomologists would typically not recommend them against this pest in the Mid-South.”

A few growers have paid the price of not heeding that advice. For example, last year about this time, Auburn University Extension entomologist Ron Smith started receiving calls from southern Alabama, south Georgia and the Florida Panhandle about growers spraying pyrethroids on tobacco budworms in conventional cotton.

“The growers wanted to do it because pyrethroids are so much cheaper than other methods,” Smith said. “In almost every case, the pyrethroids failed miserably in the field.”

The growers and/or consultants involved in the decision to treat tobacco budworms with pyrethroids may have felt that resistance may have reverted in recent years. But it hasn’t.

Today, according to Leonard, anywhere from 50 percent to 80 percent of tobacco budworms survive a discriminating dose of 10 micrograms of cypermethrin per vial. “We collect adults out of pheromone-baited wire-cone traps, place them in a small glass vial treated with insecticide.”

Smith noted that the tobacco budworm has also kept its resistance to chlorinated hydrocarbons, DDT and others, “to this day. Once they obtained it, somehow it seems to stay in the population.”

“We’re concerned,” said Martin. “But IRAC and industry is taking the approach is that the entomologists are the ones who know their area the best and can figure out the best way to use those tools and make the recommendations to restrict the use.”

Extension entomologists in Mississippi, Arkansas and Louisiana recommend that growers not use pyrethroids until after first flower, “when they’re most likely to be needed in cotton,” noted Leonard.

“Prior to that time, other types of chemistries should be used to target the specific pests that are in the field.”

For control of aphids and plant bugs prior to bloom, Leonard suggests using organophosphates, carbamates or neonicotindoids like Provado, Centric or Assail.

For tobacco budworm in non-Bt cotton, the options are new products including Steward, Tracer and Denim.

“You typically don’t have much of a problem with bollworm in cotton during June. During this time period in the Mid-South, they prefer field corn.”

Another complication for pyrethroids is that competition has driven their price down so far that cost-conscious growers can’t resist trying them. “It’s hard for a farmer to use a more expensive chemistry when he’s got this very inexpensive tool,” Martin said. “It’s a tough choice a farmer.”